The instant invention relates to electrically conductive printed circuits and more particularly to a novel method of forming an electrically conductive printed circuit on a translucent substrate.
Printed circuit boards have found wide application in many areas of today's "electronic society". In this regard, a number of different types of both rigid and flexible printed circuit boards are currently available for applications ranging from electronic game devices to space shuttle computer systems. Rigid printed circuit boards are currently used in nearly all phases of the electronic industry and in low-voltage applications they have virtually replaced the more traditional forms of circuitry wherein individual wires are connected to the various components of the circuit. So-called flexible or membrane circuit boards have recently gained wide acceptance in electrical keyboard constructions wherein a key on a keyboard is depressed to flex a particular area of a membrane circuit to cause that area to be moved into engagement with a stationary circuit element to effect electrical continuity therebetween. Membrane keyboard circuits have proven to be effective and durable in a wide range of applications and have permitted sealed keyboard constructions wherein exposure of the internal circuit elements to environmental moisture and contaminants is prevented. In keyboard constructions of this type, a number of keys with various character elements thereon are printed on a single flat flexible sheet which represents the face of the keyboard. Depression of the keys on the keyboard causes flexing of a membrane circuit to effect continuity in the manner hereinabove described.
One of the primary methods for manufacturing printed circuit boards has heretofore been to etch copper laminates utilizing conventional negative photo-resist procedures. A circuit image of a resist material is first printed on the copper laminate and acid is applied to remove the undesired portions of the copper, leaving only the portion of the copper which is in the desired circuit image. While this procedure has generally been effective for producing circuit boards of acceptable quality, it is not a completely desirable procedure in that substantial amounts of pollution frequently result from effluents that are emitted during the etching process.
A second well-known method of manufacturing printed circuits has comprised screen printing of electrically conductive inks on rigid or flexible substrates. This method has also been less than fully satisfactory in that in order to provide satisfactory flow and binding characteristics in the ink used, the inks must contain substantial quantities of binding agents. On the other hand, in order to provide sufficient circuit conductivity in the circuit, the inks must contain relatively high quantities of metallic particles. Because of these competing constraints, it has proven necessary to use highly expensive high conductive particles of precious metals, such as silver, to achieve the desired circuit conductivity and still maintain adequate binding characteristics. The high cost of silver has proven to be a major drawback in circuits of this type. In addition, it has proven necessary to use comparatively corrosive solvents in the ink which actually attack the substrate in order to achieve proper binding of the ink. Consequently, when the ink is heated to drive off the solvents, substantial quantities of corrosive pollutants are emitted. The heat required in this instance also has drawbacks in that it frequently causes unwanted substrate deformation.
The instant invention provides a novel and unique process for forming electrically conductive circuits on substrates which has none of the major drawbacks of the previously known methods. It is effective for forming durable printed circuit boards without the use of copper laminates, heat cured inks, precious metals or corrosive solvents. In particular, the method of the instant invention utilizes innovative ultraviolet photo technology to form electricaly conductive circuits on translucent substrates. An ultraviolet reactive ink which comprises a mixture of unsaturated monomers and oligomers is screen printed on a substrate in the desired circuit image. The ink is then partially polymerized or cured to an elastic or tacky state by exposing it to a limited amount of ultraviolet radiation. A wax or silicone-based resist emulsion is then screen printed on the substrate in the negative image of the desired circuit. Molecular particles of a conductive metal (which need not be a precious metal) are then sprayed on the ink by electrostatically accelerated vacuum deposition. This forms a dense crystalline layer of metallic particles on the partially cured ink. Since the particles are accelerated during the deposition thereof, a portion of them are actually embedded into the semicured ink to assure proper adhesion of the metallic layer. The substrate is then exposed to ultraviolet radiation a second time to effect final polymerization or curing of the ink. In the final curing step, ultraviolet radiation is applied to the ink from both sides of the substrate. This is done because the metallic particles on the ink tend to reflect ultraviolet radiation applied from the upper side of the substrate so that only a limited amount of radiation reaches the ink which obviously has the effect of inhibiting the curing thereof. By applying radiation to both sides of the translucent substrate, the radiation passes through the substrate and reaches the ink from the under side thereof to assist in the complete curing of the ink. After the ink has been fully cured to a hardened state, the resist material is removed from the circuit board utilizing a conventional solvent, leaving only the fully-formed circuit on the substrate.
Both the method of the instant invention and the circuit thereby formed have significant advantages over their prior art counterparts. In particular, the method of the instant invention is carried out without the use of precious metals or substantial quantities of heat. The method of the instant invention is practical and economical and produces virtually no air pollutants. The circuit boards produced are rugged and durable. Further, since the metallic layer deposited on the ink is a comparatively dense homogeneous layer, it is suitable for connection with other circuit components by soldering.
Accordingly, it is an object of the instant invention to provide a method of forming an electrically conductive printed circuit without the use of precious metals.
Another object of the instant invention is to provide a method of manufacturing electrically conductive printed circuits which produces no effluents.
A still further object of the instant invention is to provide an electrically conductive printed circuit board to which electrical components may be soldered.
An additional object of the instant invention is to provide a durable electrically conductive printed circuit board.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.